On May 23, 1991, President George H.W. Bush nominated Kleinfeld to a seat on the Ninth Circuit, vacated by Alfred T. Goodwin. He was confirmed by the Senate on September 12, 1991, and received his commission on September 16, 1991.

Kleinfeld is generally considered a conservative judge, in contrast with the more liberal circuit he sits on. But his background in private practice as a civil and sometimes criminal defense lawyer gives him a more libertarian bent, as evidenced in his opinion in Calabretta v. Floyd[1] and his dissent in U.S. v. Gourde[2]

In 2007, a Ninth Circuit panel affirmed the class action certification in Dukes v. Wal-Mart Stores, Inc., a lawsuit initiated by female employees of Wal-Mart against the company for gender discrimination. Kleinfeld wrote a sharply worded dissent, saying "this case poses a considerable risk of enriching undeserving class members and counsel, but depriving thousands of women actually injured by sex discrimination of their just due."[3]

Kleinfeld was the author of the unanimous panel decision of Morse v. Frederick, holding that a student who put up a banner supposedly supporting drug legalization was exercising his freedom of speech protected by the First Amendment, and the school principal acted unconstitutionally in suspending him. The school board appealed the decision to the Supreme Court, which heard the case on March 19, 2007.

The Supreme Court, in a 2007 majority opinion authored by Chief Justice John Roberts, reversed Kleinfeld's ruling and ruled that the First Amendment does not protect in-school student speech advocating illegal drug use.[4] One key point of disagreement between Judge Kleinfeld's opinion and Chief Justice Roberts' was whether the speech was at or during school. As the banner was displayed across the street from the school (which had been let out for the day), Judge Kleinfeld's panel held that it was an "out of school" activity. Chief Justice Roberts' majority disagreed.